Presumably the SIMs were anonymously bought/registered. There's no link to the user. If you knew the drug kingpin was using a particular sim/phone (IMSI/IEMI), you can track it, but that's like looking for a needle in a haystack (no intercepted texts/calls to go by).
If you know two locations that the kingpin in question frequents regularly (for most of us that is home & work), you can easily narrow down your haystack down to very few needles real fast. How many people live within 50m of your home and work within 50m of your office?
How does this help make the phone owner anonymous? Can't you still do triangulation with a data-only SIM?